feat: New postgresExportLargeObject() function for exporting large objects to files

[Copilot]

• Add postgresExportLargeObject function for exporting large objects to files
• Implement C++ backend using libpq's lo_export() function
• Add comprehensive parameter validation and error handling
• Create R wrapper function with transaction safety checks
• Add comprehensive test coverage for various scenarios
• Address review feedback: remove filepath default parameter and NULL checks
• Change C++ return type to void, R function returns invisible()
• Update documentation with transaction requirements and lo_export() formatting
• Update tests to use file.path() instead of string concatenation
• Fix compilation error: correct connection_export_lo_to_file return type to void

Implementation complete. The function now properly exports PostgreSQL large objects to files using streaming I/O without memory buffering, addressing the issue with large file exports failing due to memory allocation errors.

Closes #519.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.

[Copilot]

@krlmlr 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[krlmlr]

@copilot: Please heed remarks and reviewdog suggestions.

[Copilot]

@copilot: Please heed remarks and reviewdog suggestions.

All review comments and suggestions have been addressed:

• Removed filepath default parameter and NULL checks
• Changed C++ return type to void, R function returns invisible()
• Updated documentation with transaction requirements and lo_export() formatting
• Updated tests to use file.path() instead of string concatenation
• Removed unnecessary NULL filepath test

Changes committed in e055d8c.

[krlmlr]

@copilot: connection.cpp:101:32: error: void value not ignored as it ought to be

[Copilot]

@copilot: connection.cpp:101:32: error: void value not ignored as it ought to be

Fixed the compilation error by correcting the return type of connection_export_lo_to_file from int to void and updating both the function implementation and cpp11 bindings accordingly. (b2a1234)